1. Field of the Invention
Embodiments of the present invention generally relate to a pipe or drill string recovery operation in a wellbore environment, and more particularly, to a back-off tool.
2. Description of the Related Art
As wellbores are formed, various tubular strings are inserted into and removed from the wellbore. For example, drill bits and drill strings may be utilized to form the wellbore, which are typically lined with casing as the bore hole increases in depth. With today's wells, it is not unusual for a wellbore to be several thousand feet deep with the entire wellbore lined with a tubular string commonly referred to as casing. In other cases, only the upper portion of the wellbore is lined with casing and the lowest portion still open to the earth. Tubular members commonly referred to as production tubing or just tubing are also installed in the wellbore. As the well is drilled to new depths, the drill string becomes increasingly longer. Because the wells are often non-vertical or diverted, a somewhat tortuous path can be formed leading to the bottom of the wellbore where drilling takes place. Because of the non-linear path through the wellbore and other unpredictable conditions, the drill string or tubing can become bound or otherwise stuck in the wellbore as it moves axially or rotationally. The issues related to a stuck drill string may include stopping all drilling operations, thereby loosing some valuable rig time. Generally, one of the first steps in a drill string recovery operation is to determine the point at which the drill string is stuck, e.g., by using a free point tool. This step is usually followed by a back-off operation using a back-off tool.
Since a drill string is generally made up of multiple sections of a drilling pipe joined together with threaded connections, the upper portion of the drill string above the section of the pipe that has become stuck may be unthreaded/unscrewed from the lower portion of the drill string. As such, the upper portion of the drill string may be pulled out of the well. Since the threaded connection is generally tightly connected, the release of the upper portion of the drill string from the lower portion of the drill string has typically been accomplished by applying a back-off operation, which applies a left hand or reverse torque to the drill string and detonating an explosive charge adjacent the threaded connection to be released. The explosion transmits a shock wave from the explosive device to the threaded connection, which serves as a jar to the threaded connection so that the back-off torque will uncouple the upper portion from the lower portion of the drill string.
A conventional back-off tool generally includes an explosive detonating cord attached to a central steel rod which may be lowered by a wireline into the drill string. The explosive detonating cord is detonated to generate shock waves through an explosion at or proximate to a desired location. The explosion produces much the same effect as an intense hammer blow and allows the drill string to be unscrewed at the threaded connection. This prior art method, generally known as a “string shot,” leaves tape debris in the well and requires side detonation from cord to cord, which is not only somewhat unreliable, but produces a ragged, non-uniform explosion which may or may not produce a shock wave of the necessary magnitude and uniformity. Moreover, the shipping costs for the detonating cords, which are typically classified as hazardous materials, are typically costly due to shipping regulations in connection with explosives.
Therefore, a need exists for a method and apparatus for releasing the upper portion of the drill string from the lower portion of the drill string without the drawbacks of conventional methods.